Postnatal growth and repair of skeletal muscle requires adult muscle stem cells called satellite cells. Located between the myofiber plasma membrane and the basal lamina, satellite cells are generally maintained as a mitotically quiescent population until stimulated to activate and exit quiescence. During activation, satellite cells upregulate MyoD, a master regulatory transcription factor in muscle. However, the mechanisms governing exit from quiescence and subsequent MyoD expression are poorly understood. Gene expression profiling of in vivo satellite cell activation revealed significant depletion of transcripts encoding proteins involved in mRNA decay, strongly implicating post-transcriptional mRNA regulation in satellite cell activation. In this proposal, I present evidence that one of these mRNA decay factors, Tristetraprolin (TTP), suppresses satellite cell activation by destabilizing MyoD mRNA. Thus, post-transcriptional mRNA regulation by TTP may play an important role in the steady-state maintenance and re-acquisition of satellite cell quiescence required for satellite cell self-renewal following activation. This proposal will address: 1) TTP/Tis11-family function during muscle regeneration, 2) mRNA targets mediating TTP/Tis11-family function during satellite cell activation, and 3) corruption/rewiring of this homeostatic network during sarcoma initiation and progression. These research aims are coupled with hands-on training components in advanced mouse genetics, RNA biochemistry, and bioinformatics, and complement my previous experiences studying cancer (graduate school) and adult stem cell signaling (prior postdoctoral training). The opportunities outlined in this proposal are designed to provide me with a multidisciplinary conceptual background and experimental toolset tailored towards independent study of post-transcriptional mechanisms of stem cell activation in health and disease. An exceptional research environment at the University of Colorado-Boulder as well as close mentorship by/collaboration with experts in muscle and stem cell biology (Drs. Olwin, Leinwand and Yi), mRNA biochemistry (Dr. Roy Parker) and pre-clinical mouse sarcoma models (Dr. David Kirsch), will enable successful completion of these research endeavors. Ultimately, the research and training plan outlined in this proposal will lay a foundation for the establishment of an academic research career dedicated to understanding fundamental mechanisms of muscle stem cell homeostasis and how these processes are deregulated in disease.